<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<div id="app">
    <input v-model="username">
        {{username}}
</div>
<body>
<script src="js/vue.js"></script>
<script>
    new Vue({
        el:"#app",
        data() {
            return {
                username:""
            }
        }
    })
</script>
</body>
</html>